Why NASA's James Webb Telescope will study gas giants before looking for aliens

In the latest addition to NASA's toolkit in its search for habitable planets larger than Earth, called “super-Earths”, is the Transiting Exoplanet Survey Satellite (TESS). Launched in April 2018, its main objective is to scan regions of the cosmos just outside our solar system for planets that could support life.

At the slightest blip towards a potential lead that TESS discovers, NASA's space observer — the James Webb Telescope — has seen more than one delay in its estimated launch. It was recently committed to launching by the end of 2021, and will be NASA's on-call eye to peek into what TESS deems as a life-supporting atmosphere.

What Webb and TESS are primed to do

The missions lined up for the Webb Telescope are a mix of studies to give NASA new insight into space objects TESS finds, and to test the capabilities of Webb’s science instruments. Since observing small exoplanets with thin atmospheres like Earth's will be challenging for Webb, astronomers will target easier, gas giant exoplanets first, according to an official statement.

The foremost struggle in the hunt for exoplanets is that the light from a star is very bright, bright enough to dwarf a faint light from potentially habitable planets nearby. Till NASA gets a better understanding of exoplanets to overcome this challenge, its scientists have armed their exoplanet project with a temporary fix.

The Webb Telescope will be primed to look for planets orbiting red dwarf stars, which are the smallest of stars weighing between 7.5 to 50 percent of our sun's mass. Since these red dwarfs are smaller and dimmer through a telescope, astronomers believe it could tease out a signal from a star-orbiting planet with relative ease. Red dwarfs are also the most common stars in our galaxy, which improves the odds of locating an orbiting exoplanet capable of supporting life.

An artistic rendition of the James Webb Space Telescope. Image courtesy: NASA

How the Webb Telescope works

When a planet crosses in front of, or transits, its host star, the star’s light is filtered through the planet’s atmosphere. Molecules within the atmosphere absorb certain wavelengths, or colours, of light. By splitting the star’s light into a rainbow spectrum, astronomers can detect those sections of missing light and determine what molecules are in the planet’s atmosphere.

The project team has already zeroed in on a prospect. WASP-79b, a planet the size of Jupiter about 780 light-years away from Earth will be first of many targets for the Webb telescope. The team that will study the telescope's observation expect to find and quantify water, carbon monoxide, and carbon dioxide on WASP-79b. NASA also thinks it could host previously unknown molecules, which they see as a likely finding in exoplanet atmospheres.

“We have two main goals. The first is to get transiting exoplanet datasets from Webb to the astronomical community as soon as possible. The second is to do some great science so that astronomers and the public can see how powerful this observatory is,” said Jacob Bean of the University of Chicago, a co-principal investigator on the transiting exoplanet project.

Webb's first big mission

Astronomers manning the Webb telescope intend to observe the entire orbit of the exoplanets it surveys to map its temperature, clouds, and chemical variations along its width, a NASA statement said.

Planets orbiting their stars in very close proximity tend to be what scientists called 'tidally-locked'. In such cases, one side of a planet is permanently facing the star it orbits, with the other side faces away. This is akin to the case of our Moon, one hemisphere of which always faces the Earth. In the case of planets tidally-locked with stars, anyone observing it through a telescope would the cooler face of the planet. On longer observation, the planet will show more of its hotter, day-facing surface as it orbits the star.

“We have already seen dramatic and unexpected variations for WASP-79b with Hubble and Spitzer. With Webb we will reveal these variations in significantly greater detail to understand the physical processes that are responsible,” said Jacob Bean of University of Chicago, currently a co-principal investigator on the exoplanet project.

The project's long-term goals

“TESS should locate more than a dozen planets orbiting in the habitable zones of red dwarfs, a few of which might actually be habitable. We want to learn whether those planets have atmospheres and Webb will be the one to tell us,” Kevin Stevenson of the Space Telescope Science Institute, another co-principal investigator on the project, was quoted as saying.

He added, “The results will go a long way towards answering the question of whether conditions favourable to life are common in our galaxy.”